A cutting insert according to one embodiment of the present invention is provided with: an upper surface having, provided to edges thereof, first upper cutting edges and second upper cutting edges; a lower surface having, provided to edges thereof, first lower cutting edges and second lower cutting edges; and side surfaces positioned between the upper surface and the lower surface. The side surfaces comprise: first surfaces which extend from the second upper cutting edges; second surfaces which extend from the first lower cutting edges; third surfaces which extend from the first upper cutting edges; and fourth surfaces which extend from the second lower cutting edges. The first surfaces and the fourth surfaces are parallel with respect to a central axis. The second surfaces are slanted so as to move away from the central axis as said second surfaces extend from the lower surface to the upper surface. The third surfaces are slanted so as to move away from the central axis as said third surfaces extend from the upper surface to the lower surface.